In the Bayer process, bauxite is contacted with recycled caustic aluminate liquor at elevated temperatures and pressures to extract the alumina content of the bauxite. The resulting slurry thus contains dissolved alumina and undissolved red mud (i.e., iron oxides, silicates, titanium oxide, etc.). The red mud is separated leaving a clear caustic aluminate solution known as "pregnant liquor" which is then seeded with aluminum trihydroxide to precipitate approximately half of the dissolved alumina content. The precipitated aluminum trihydroxide is then separated from the caustic aluminate solution. A portion of the precipitated aluminum trihydroxide is recycled to be used as seed for subsequent precipitation of aluminum trihydroxide and the remainder is recovered as product. The remaining caustic aluminate solution (hereinafter referred to as "spent liquor") is either recycled in the process for further alumina recovery as it is, or is in part concentrated by evaporation prior to recycling to the bauxite extraction step.
The bauxite used in the Bayer process contains organic substances which dissolve wholly or in part during bauxite digestion. The organic substances degrade to lower molecular weight compounds under the influence of the high caustic concentration and elevated temperatures experienced during bauxite digestion. Thus, the Bayer liquor may include various organic carbon compounds ranging from high molecular weight humic-type compounds to final degradation products such as sodium oxalate.
Sodium oxalate presents a special organics problem in that it is the only one of the many degradation products formed which accumulates to a concentration exceeding its solubility in solution. Caustic aluminate solutions are thus supersaturated with respect to sodium oxalate and are, to some extent, stabilized in this condition by the presence of the other organic carbon compounds in solution.
Sodium oxalate is thus a major impurity in the caustic aluminate liquor of the Bayer process. As long as it stays dissolved in solution, sodium oxalate is considered to be relatively harmless. Difficulties arise however when the dissolved sodium oxalate crystallizes at the temperatures and caustic concentrations employed at the end of the precipitation cycle of the Bayer process, i.e., the precipitation of product aluminum trihydroxide. The crystalline sodium oxalate which forms can interfere with particle agglomeration and stimulate the formation of fine new crystals of aluminum trihydroxide. Thus, the presence of crystalline sodium oxalate has a deleterious effect on the particle size of the product aluminum trihydroxide. Moreover, the presence of crystalline sodium oxalate can cause difficulties in the filtration of aluminum trihydroxide slurries.
A characteristic of modern alumina plants is that the aluminum trihydroxide precipitation circuit is divided into two parts. In the first part, the finer aluminum trihydroxide particles are subjected to relatively rapid size enlargement by an agglomeration mechanism whilst in the second part the agglomerates are consolidated into strong particles with crystal growth the main operating mechanism. The aluminum trihydroxide particles subjected to the agglomeration process are generally washed free of any crystalline sodium oxalate prior to precipitation, whereas aluminum trihydroxide entering the growth precipitators (i.e., agglomerated particles + recycled coarse material) is not subjected to any washing procedure.
The water used to wash aluminum trihydroxide seed and product particles can contain significant amounts of dissolved sodium oxalate. Typically, aluminum trihydroxide in the unwashed condition may contain 0.1-1.0% sodium oxalate, with respect to the overall quantity of aluminum trihydroxide. By concentrating wash waters by wash-water evaporation, it is possible to subsequently recrystallize the sodium oxalate which can then be separated and disposed of. Thus, seed washing constitutes a recognized method of removing sodium oxalate from the Bayer process (see, for example, Roberts et al., U.S. Pat. No. 3,372,985). It can be appreciated, however, that sodium oxalate removal by the, `Seed Washing`, process can only function when crystalline sodium oxalate is already present in the aluminum trihydroxide precipitation circuit. In other words, the aluminum trihydroxide precipitation circuit must be, to some degree, in difficulty before `Seed Washing` can work.
Other techniques have been developed and are used for removing sodium oxalate from the caustic aluminate liquors of the Bayer process. These generally involve seeding systems of some type and exploit the well-known sodium oxalate solubility relationships, particularly the temperature and caustic concentration dependencies (see, for example, Sato et al., U.S. Pat. No. 3,649,185 and Fujiike et al., French Patent No. 2,405,901).
Probably the most effective approach, at least in terms of consistently removing sufficient sodium oxalate from the process to maintain sodium oxalate, in both the dissolved and solid states, at acceptably low levels, is side-stream crystallization of sodium oxalate by seeding partially concentrated spent liquor by liquor evaporation, as opposed to wash-water evaporation (see, for example, Carruthers et al., U.S. Pat. No. 4,038,039 and Yamada et al., U.S. Pat. No. 4,263,261). The long-standing problem of the progressive contamination and deactivation of the sodium oxalate seed crystals by the other organic carbon compounds present in solution has been overcome by the employment of a suitable wash process which regenerates the activity of the seed crystals and maintains a crystal form suitable for filtration and separation of the crystalline sodium oxalate (see, for example, Yamada et al., Light Metals Conf. Proceedings (1973) 745-754). Alternatively, techniques have been developed for removing the harmful organic contaminants prior to seeded crystallization of sodium oxalate (see, for example, Gnyra, U.S. Pat. No. 4,275,043 and Lever, U.S. Pat. No. 4,275,042).
Thus, side-stream crystallization of sodium oxalate from caustic aluminate liquor suitably concentrated by partial evaporation provides an attractive alternative to the `Seed Washing` process for sodium oxalate removal. Not all alumina plants, however, are equipped with liquor evaporators (in itself a desirable goal due to the energy intensive nature of liquor evaporation). This applies particularly to alumina plants operating the tube digestion system.
Therefore, what is needed in the art is a method for sodium oxalate removal which achieves at least the equivalent to that of seeded crystallization of partially evaporated liquor without the need to preconcentrate using liquor evaporation.